1. Technical Field
This invention relates to the field of medical introducer apparatuses. More particularly, the invention relates to a method of forming an introducer sheath, and to an introducer sheath having a crease-free inner diameter formed by the inventive method.
2. Background Information
Numerous advances of considerable note have occurred in medical surgical techniques over the last few decades. Among the most significant advances has been the adoption, and now-routine performance, of a variety of minimally invasive procedures. Such procedures include angioplasty, endoscopy, laparoscopy, and arthroscopy, as well as numerous other diagnostic and therapeutic operations. These minimally invasive procedures can be distinguished from conventional open surgical procedures in that access to a site of concern within a patient is achieved through a relatively small incision, into which a tubular device (or tubular portion of a device) is inserted or introduced. The tubular device, or device portion, keeps the incision open while permitting access to the target site via the interior (i.e., the lumen) of the tube.
Body passageways in which introducer apparatuses have been used to introduce medical interventional devices and/or liquid medicaments include the esophagus, trachea, colon, biliary tract, urinary tract, and virtually all portions of the vascular system, among others. One particularly significant example of a minimally invasive technique involves the temporary or permanent implantation of a medical interventional device, such as a stent, into a body passageway of a patient. Other examples involve the transmission of a liquid medicament to a target area, and/or the withdrawal of body fluid from the body passageway.
When carrying out these, and other, desired techniques, communication with the passageway is typically attained by inserting an access device, such as an introducer sheath, into the body passageway. One typical procedure for inserting the introducer sheath is the well-known Seldinger percutaneous entry technique. In the Seldinger technique, a needle is initially injected into the passageway, such as a vessel, and a wire guide is inserted into the vessel through a bore of the needle. The needle is withdrawn, and an introducer assembly is inserted over the wire guide into the opening in the vessel.
Typically, the introducer assembly includes an outer introducer sheath, and an inner dilator having a tapered distal end. The tapered end of the dilator stretches the opening in the vessel in controlled fashion, so that introduction of the larger diameter introducer sheath may then be carried out with a minimum of trauma to the patient. Following satisfactory placement of the introducer sheath, the dilator is removed, leaving at least the distal portion of the larger diameter introducer sheath in place in the vessel. The interventional device, such as a stent, etc., or the liquid medicament may then be passed through the introducer sheath for delivery to the target site.
Historically, percutaneous insertion techniques were problematic, due in large part to the lack of flexibility and/or kink resistance of the sheath. Early sheaths were generally formed of a relatively stiff fluoropolymer, such as polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP). The sheaths were typically of thin-walled construction, and were prone to kinking, particularly when threaded through tortuous pathways within the body. Increasing the thickness of the sheath only minimally improved the kink resistance of the sheath. At the same time, the added thickness occupied valuable space in the vessel, thereby minimizing the diameter of the interventional device that could be passed therethrough. In addition, increasing the thickness of the sheath necessitated the use of a larger entry opening than would otherwise be required.
A kinked sheath is essentially unusable, and generally cannot be straightened while positioned in the body of the patient. Consequently, once a sheath kinks, the sheath must be removed, leaving an enlarged, bleeding opening which typically cannot be reused. Access to the vessel must then be re-initiated at an alternative site, and the process repeated with a new sheath. In many cases, a suitable alternative site is not available, and the percutaneous procedure must be abandoned altogether in favor of a different, and often more intrusive, technique.
In recent years, introducer sheaths have been improved in order to enhance their flexibility and kink resistance. Such sheaths are now routinely used to percutaneously access sites in the patient's anatomy that previously could not be accessed with existing sheaths, or that could be accessed only upon the exercise of an undesirable amount of trial and error, with the concomitant discard of sheaths whose placement had been unsuccessful.
Many newer sheaths exhibit a much higher degree of kink resistance than was achievable with prior art sheaths. One example of a flexible, kink resistant introducer sheath is described in U.S. Pat. No. 5,380,304. The sheath described in this patent includes a lubricious inner liner having a helical coil fitted over the liner. An outer tube is connected to the outer surface of the liner through the coil turns. The coil reinforcement imparts kink resistance to this thin-walled sheath through a wide range of bending.
U.S. Patent Publication No. 2001/0034514 discloses an introducer sheath similar in many respects to the sheath of the '304 patent. The sheath in the patent publication is formed such that the proximal end of the sheath has a higher stiffness, while the distal end has a lower stiffness. Since the distal portion of the sheath has a lower stiffness (and therefore is more flexible) than the proximal portion, the sheath is able to traverse portions of the anatomy that would have been difficult, if not impossible, to traverse with stiffer sheaths. Since the proximal portion has a higher stiffness (and is therefore less flexible) than the distal portion, the sheath maintains the trackability to traverse tortuous areas of the anatomy. This presence of the coil reinforcement also enables this sheath to be kink resistant through a wide range of bending angles.
U.S. Pat. No. 6,939,337 discloses a sheath having a coil reinforcement, as well as a braid reinforcement positioned over (i.e., radially outwardly of) at least a length of the coil. This sheath utilizes a coil for the purposes of providing kink resistance in the same manner as the '304 patent and the patent publication recited above, and also includes a braid to enhance torqueability and pushability of the sheath. Each of the patent references cited above is incorporated herein by reference.
The development of introducer sheaths, such as those described above, has revolutionized the practice of medicine. In particular, this development has enhanced the ability of the physician to introduce medical interventional devices and liquid medicaments into target sites that had previously been difficult, if not impossible, to reach without the necessity of carrying out much more intrusive open surgical operations. The percutaneous methods described are generally less expensive than the open surgical methods previously employed, are less traumatic to the patient, and typically require a shorter patient recovery time.
Notwithstanding the benefits that have been achieved by the use of such introducer sheaths, new challenges continue to be faced. For example, as noted above, introducer sheaths are frequently introduced into the body passageway in combination with a tapered inner dilator. In many cases, the introducer sheath is provided with a tapered distal end that corresponds, in some fashion, to the taper of the dilator. This relationship is intended to minimize the transition between the dilator and introducer sheath upon insertion of the sheath through the percutaneous opening, thereby further minimizing the trauma experienced by the patient upon insertion.
Many typical introducer sheaths, such as the sheaths described in the incorporated-by-reference patents, may include a lubricious inner liner formed of a relatively stiff fluoropolymer, such as PTFE. The reinforcement, e.g., the coil and/or braid, is fitted over the liner, and the outer tube, formed of a more flexible polymer, such as a polyether block amide, nylon, or polyurethane is fitted over the liner. The entire assembly is typically placed in a heat shrink enclosure, and heated in an oven. The outer surface material melts and bonds to the outer surface of the inner liner through the coil turns, or the filaments of the braid.
Typically, such sheaths have a constant, or substantially constant, inner diameter extending virtually the entire length of the sheath, from the proximal end to the distal tip area. However, as stated above, in some cases it is desired to taper the distal tip to correspond to the taper of the dilator. In this event, the distal tip portion of the sheath is generally placed in a heated die that gradually reduces the diameter of the distal tip to the desired taper. Since the inner liner is generally formed of a higher melting material (e.g., PTFE) when compared to the material of the outer jacket (e.g., a polyether block amide), the liner does not melt flow in the die in the nature of the outer jacket under the conditions in the die. As a result, as the diameter of the sheath decreases upon tapering, creases or wrinkles are prone to form along the inner diameter of the liner.
It is desired to overcome the problems of the prior art by providing a method of forming an introducer sheath having a tapered distal tip formed by the inventive method, wherein the inner diameter of the sheath is substantially free of wrinkles and creases at the tapered distal tip portion. It is also desired to provide an introducer sheath having an inner liner substantially free of wrinkles and creases.